For this application, this method has recently been employed in self-clocking systems made of integrated circuits and using standardized high-speed transmissions over a fiber distributed data interface (FDDI). In the transmitter, a clock signal on the order of 25 MHz controls the division of each eight-bit data byte into two groups of four bits. Re-encoding converts each group of four data bits into one group of five bits containing no more than a predetermined number n of consecutive zeros in accordance with the known RLLn code; this number in practice is three, in the RLL3 code. The transmission signal is made up of the serial connection of all the re-encoded groups. The re-encoding is thus a kind of incorporation of clock information into the serial data transmission signal. On the other hand, the biunivocal correspondence between an original group and the encoded group leaves numerous encoded groups without matches, among which one is selected as synchronizing information. This synchronizing information is also incorporated into the serial transmission signal. Multiplication of the frequency of the clock signal produces the transmission frequency. With a multiplication factor of 40, the transmission speed can reach 1 Gbps. The transmission signal is encoded in the NRZI mode (inverted non-return to zero). In this mode, each logical "1" is translated into a level-changing edge of the encoded signal, while a logical "0" preserves the level of the encoded signal. This signal is transmitted to a receiver via a transmission link. In the receiver, the clock information contained in the transmission signal received is recovered first. The recovery is done by selective Fourier spectral analysis of the digital signal representing each encoded group. The frequency of the restored clock signal is multiplied to constitute the frequency of the transmission signal and consequently to decode the groups received In practice, given the random presence of data edges in the transmission signal, the recovery circuit includes a resonator. In the absence of edges, the recovery circuit fixes on the harmonic of the signal entering the range of the resonator, to constitute an apparent reference edge. Sampling of the data is done with respect to the actual and apparent edges of the transmission signal received On the other hand, the synchronizing information is picked up from the transmission signal received It is used only once, to frame the eight-bit bytes of the output signal of the receiver.
This transmission method has several major disadvantages. Re-encoding to perform the spectral analysis of the transmitted signal means a loss in data transmission speed. In the RLL3 code for re-encoding half of an eight-bit byte into one group of five bits, the most favorable case for spectral analysis corresponds to a group containing nothing but logical "1"s. Thus the transmission signal has one edge for each bit of the group and has a fundamental component corresponding to twice the recurrence R of the bits transmitted. The clock component detected by the selective Fourier spectral analysis of the transmission signal received is extracted by filtration at the frequency 1/R. The least favorable case occurs in the absence of an edge for three successive periods R. In that case, the frequency of the fundamental component is 1/3R, the second-order frequency is 2/3R and the third-order component corresponds to the fundamental component 1/R of the preceding case. However, the third-order harmonic has a markedly smaller amplitude than the fundamental component. It follows that RLLn re-encoding is limited to low values of n. For example, a component of order 9 to be filtered would be very weak and would be very close to its neighboring components of frequencies 8/9R and 10/9R. Because of the low amplitude and the size of these three components, and above all because of deviations in the electrical characteristics of the components, such filtering would prove impossible to actually perform. In conclusion, the re-encoding that is necessary in practice is a very major constraint and limits the data transmission speed.
Another disadvantage of the current method resides in the insertion of synchronizing information into the transmission signal. This information is added to the clock information introduced into the transmission signal in the form of re-encoding of the halves of eight-bit bytes. Furthermore, this information comprises an encoded group that does not correspond to a datum and is used only a single time. On this occasion, synchronization done under poor conditions does not enable restoration of the data upon reception.